The Mystic Paintbrush is a magical paintbrush and an attack performed by Kwazii when he uses his Humphead
Mystic paint brush by cmanuel1-d8jwvxd

Kwazii and his mystic paintbrush

Parrotfish charm on his ocean pearl bracelet.

Like Amu Hinamori's attack as Amulet Spade, Colorful Canvas, he uses the paintbrush's paint by spinning it in his paw and throwing out rainbow colored paint. This also serves to trap anything in the paint. 

However, Kwazii can also use the Mystic Paintbrush to paint anything he wants and then turn it real. And in Paint Me A Prediction, he can use the paintbrush to paint predictions that see to come true.

The phrase goes by "Here's paint in yer' eyes!"